The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery having a large capacity with improved charge/discharge characteristics.
Recent development of devices in the electronic field is remarkable, and various devices such as video cameras, liquid crystal cameras, portable telephones, laptop computers, and word processors are now being developed. In accordance therewith, there are increasing needs for reduction in size and weight and achievement of high energy density in batteries that are to be used as power sources for these electronic devices.
Conventionally, lead batteries or nickel-cadmium batteries have been used for these electronic devices. These conventional batteries cannot sufficiently meet the needs for reduction in size and weight and achievement of high energy density.
Accordingly, there are proposed non-aqueous electrolyte batteries using a non-aqueous electrolytic solution containing a lithium salt dissolved in a non-aqueous solvent. As these non-aqueous electrolyte batteries, batteries in which a metal lithium, a lithium alloy, or a carbon material capable of being doped and undoped with lithium ions is used as a negative electrode material and a lithium cobalt composite oxide is used as a positive electrode material are already in practical use.
Having a high operation voltage of 3 to 4 V, the non-aqueous electrolyte batteries of this type can be made to have a high energy density and excellent cycle characteristics with only a small amount of self-discharge.
Also, in order to attain further reduction in size and weight and achievement of high energy density in these non-aqueous electrolyte batteries, eager researches for development of active materials and the like are now being carried out. As positive electrode active materials, Ni-containing lithium composite oxides such as lithium-nickel composite oxides and lithium-nickel-cobalt composite oxides are also proposed.
Meanwhile, Japanese Laid-open Patent Publication No. 5-290847/1993, for example, discloses use of Li1+x CoO2 as a positive electrode active material to provide lithium corresponding to the latent capacity thereof for precharging the negative electrode so as to increase the battery capacity. However, the battery capacity has not been sufficiently increased.
Objects of the Invention
On examining the capacity and the charge/discharge characteristics of a non-aqueous electrolyte secondary battery, the present inventors have found that it is not possible to improve the capacity and the charge/discharge characteristics of the battery as designed even if each of the capacities of the positive and negative electrodes is simply increased.
Accordingly, the object of the present invention is to solve the above-mentioned problems of the prior art and to provide a non-aqueous electrolyte secondary battery having a large capacity with improved charge/discharge characteristics.
Summary of the Invention
The present inventors have made an eager research and found out that a battery can have increased capacity and improved charge/discharge characteristics by observing each of the initial efficiencies of positive and negative electrodes and by combining the positive electrode and the negative electrode so that the initial efficiencies of the positive and negative electrode satisfy a specific relationship, thereby completing the present invention.
Accordingly, the present invention provides a non-aqueous electrolyte secondary battery including a positive electrode and a negative electrode each capable of being doped and undoped with lithium ions, wherein the positive electrode and the negative electrode are combined so that the relationship:
0.9xe2x89xa6Kp/Knxe2x89xa61.1
is satisfied, where an initial efficiency of the positive electrode is represented by Kp and an initial efficiency of the negative electrode is represented by Kn.
In the present invention, the initial efficiency Kp of the positive electrode is a ratio of a discharge capacity to a charge capacity when it is first charged to 4.2 V and then discharged to 3.0 V using lithium as a counter electrode. In other words,
Kp=(initial discharge capacity)/(initial charge capacity).
The initial efficiency Kn of the negative electrode is a ratio of a charge capacity to a discharge capacity when it is first discharged to +0.0 V and then charged to 2.0 V using lithium as a counter electrode. In other words,
Kn =(initial charge capacity)/(initial discharge capacity).
Although nickel-containing lithium composite oxide as an active material has a large capacity, its initial efficiency tends to be poorer than cobalt lithium oxide. Also, hard carbon and polymer carbon tend to have a larger capacity but with a poorer initial efficiency than an active material of graphite. In a conventional method, if the initial efficiencies of the positive electrode and the negative electrode are different, the one having the poorer initial efficiency is packed in a smaller amount to fabricate a battery, or alternatively the one having the better initial efficiency is loaded. If an attempt is made to increase the capacity, the initial charge/discharge capacities tend to be poorer. Conventionally, an improvement to increase the initial efficiency has been made. Also, in a conventional lead battery, for example, the concentration of an electrolyte in an electrolytic solution changes by charging and discharging. In a battery where electricity is produced by anions and cations in the electrolyte reacting with the positive electrode and the negative electrode, the capacity changes greatly according to the initial efficiency. Therefore, increased initial efficiency contributes to increase in the capacity.
However, in a lithium ion secondary battery, electricity is taken out by means of lithium ions moving between the positive electrode and the negative electrode, so that basically the concentration of the electrolyte in the electrolytic solution does not change. In other words, the electrolyte in the electrolytic solution is not consumed by charging or discharging. Therefore, it has been found out that it is not necessary to increase the initial efficiency with efforts but, rather, the initial efficiency ratio between the positive electrode and the negative electrode is important. Thus, the present invention provides a battery having a large capacity with improved charge/discharge characteristics by allowing the ratio of the initial efficiencies of the positive and negative electrodes to be within the above-mentioned specific range.
In the present invention, the active material of the positive electrode preferably contains a lithium composite oxide having a composition of LixNiyMzO2 (where x satisfies 0.8 less than x less than 1.5, y+z satisfies 0.8 less than y+z less than 1.2, z satisfies 0xe2x89xa6z less than 0.35, and M is at least one element selected from Co, Mg, Ca, Sr, Al, Mn and Fe).
Detailed Description of the Invention
In the present invention, a lithium composite oxide is used as an active material of the positive electrode. Examples of the lithium composite oxides to be used include LixCoO2 (0 less than xxe2x89xa61.0), LixNiO2 (0 less than xxe2x89xa61.0), Li1+XMn2xe2x88x92XO4 (0xe2x89xa6xxe2x89xa6⅓), Li(M, Mn)2O4 (M=Cr, Co, Al, B), and others.
In the present invention, it is particularly suitable in obtaining large capacity with low costs that the lithium composite oxide is LixNiyMZO2 (where x satisfies 0.8 less than x less than 1.5, y+z satisfies 0.8 less than y+z less than 1.2, z satisfies 0xe2x89xa6z less than 0.35, and M is at least one element selected from Co, Mg, Ca, Sr, Al, Mn and Fe). In this case, the metal M is more preferably Co, and may be two or more kinds of the metals.
An example of a method for producing such a lithium composite oxide is, for example, a process in which a basic metal salt and an alkaline water-soluble lithium compound containing respectively an anion that volatilizes at the time of calcination of LiMetal3+O2 (where the Metal contains Ni as a major component and further contains at least one element selected from Co, Mg, Ca, Sr, Al, Mn,and Fe) are allowed to react in an aqueous medium to obtain a slurry, which is then dried and calcined.
The basic metal salt is represented by the general formula: Metal2+(OH)2xe2x88x92nk(Anxe2x88x92)kxc2x7mH2O. Here, the Metal2+is an ion containing Ni as a major component and possibly containing at least one element selected from Co, Mg, Ca, Sr, Al, Mn and Fe. Anxe2x88x92represents an anion with n valences (where n=1 to 3) such as a nitrate ion, a chloride ion, a bromide ion, an acetate ion, or a carbonate ion. Further, k satisfies 0.03xe2x89xa6kxe2x89xa60.3; and m satisfies 0xe2x89xa6m less than 2.
The basic metal salt represented by the above-mentioned formula can be produced by adding to an aqueous solution of Metal2+an alkali of about 0.7 to 0.95 equivalent, preferably about 0.8 to 0.95 equivalent, relative to the Metal2+, and reacting them under a reaction condition of about 80xc2x0 C. or less, and then maturing the reaction product at a temperature of 40xc2x0 C. to 70xc2x0 C. for 0.1 to 10 hours, followed by washing with water to remove the by-products. The alkali to be used in the reaction may be a hydroxide of an alkali metal such as sodium hydroxide, a hydroxide of an alkali earth metal such as calcium hydroxide, an amine, or the like.
A basic metal salt selected from the compounds represented by the above-mentioned formula and one or more lithium compounds selected from lithium hydroxide, lithium carbonate, hydrates thereof, and the like are allowed to react in water at a concentration in the range of 5 to 25 wt % and at a temperature in the range from room temperature to 100xc2x0 C. to obtain a slurry, which is then subjected to spray drying for improvement of uniformity in the shape of the composition to be obtained.
The lithium composite oxide can be obtained by subjecting the dried product to a thermal treatment for calcination in an oxidizing gas atmosphere containing air, oxygen, ozone, or the like in a temperature range of about 700 to 1000xc2x0 C. for about 0.1 to 20 hours.
Another example of a method for producing a lithium composite oxide to be used in the present invention is a process that uses a water-soluble lithium compound and a basic metal carbonate obtained from a water-soluble metal compound.
The water-soluble metal compound to be used in this process is a nitrate, a sulfate, a metal chloride, or the like. This water-soluble metal compound may contain a nickel compound as a major component and may be mixed with a given amount of another water-soluble metal compound so that at least one element selected from Co, Mg, Ca, Sr,, Al, Mn and Fe may be blended therewith.
The basic metal carbonate may be obtained by filtrating and drying a precipitate obtained by allowing a mixture of the above-mentioned water-soluble metal compounds to react with a compound selected from the group consisting of an alkali carbonate, an alkali bicarbonate, ammonium carbonate and ammonium bicarbonate in water, or a precipitate obtained by allowing sodium hydroxide to be present for reaction in the above-mentioned reaction system. In this case, in order to produce a good precipitate, it is preferable to use a little excessive amount of the carbonate, and also it is important to control the stirring condition so as to control the specific surface area of the precipitate.
To the basic metal carbonate thus obtained, a powder of a water-soluble lithium compound such as lithium carbonate or lithium hydroxide is added at a desired ratio of the metal to Li. The resultant mixture in a powder state is first heated to 300 to 500xc2x0 C. in the presence of an inert gas or an oxygen-containing gas. This heating allows only the decomposition of the basic metal carbonate to proceed, whereby carbonic acid gas in the crystal structure is released. This heating is continued until the generation of the carbonic acid gas substantially stops so as to convert all of the basic metal carbonate into a metal oxide having numerous fine pores.
After the generation of carbonic acid gas substantially stops, the temperature is further raised to allow the molten water-soluble lithium compound to penetrate into the fine pores of the metal oxide, whereby the two compounds will be in an extremely close contact. At this moment, the resultant product is calcined at a temperature of 700 to 900xc2x0 C. in the presence of oxygen gas or an air rich in oxygen, whereby Ni is turned from bivalent to trivalent to produce a Li composite oxide.
Here, the larger the specific surface area of the basic metal carbonate to be used is (for example, more than 100 m2/g), the more preferable it is, because gas discharge and generation of fine pores after preliminary calcination will be more efficiently performed.
A positive electrode mixture-coating material is prepared by kneading such a positive electrode active material, an electrically conductive agent such as acetylene black or graphite, and a binder such as polytetrafluoroethylene or polyvinylidene fluoride together with an organic solvent such as N-methyl-2-pyrrolidone. The coating material is applied onto a collector such as aluminum foil and dried to obtain the positive electrode. The electrically conductive agent, the binder, the organic solvent and the collector are not specifically limited, and may be selected from a variety of materials.
Now, the negative electrode active material to be used in combination with such a positive electrode active material will be hereafter explained.
The negative electrode active material is not specifically limited and may be any material capable of being doped and undoped with lithium, a lithium alloy, or lithium ions. Such a material may be a carbon material, tin oxide, or the like. Examples of the materials include graphites, glassy carbons, polymer carbons which are carbon materials obtained by thermally treating a high polymer having a cross-linked structure in an inert atmosphere (hard carbons obtained by carbonization of a synthetic resin such as cellulose, phenolic resin, furfural resin, polyparaphlenylene or polyacrylonitrile), and others. Especially, polymer carbon is suitable because it has a large capacity.
A negative electrode mixture-coating material is prepared by mixing and/or kneading such a negative electrode active material, an electrically conductive agent, and a binder together with an organic solvent. This coating material is applied onto a collector such as copper foil and dried to obtain the negative electrode. The electrically conductive agent, the binder, the organic solvent and the collector are not specifically limited, and may be selected from a variety of materials.
In the present invention, in combining the positive electrode and the negative electrode, it is necessary that the relationship: 0.9xe2x89xa6Kp/Knxe2x89xa61.1 is satisfied, where the initial efficiency of the positive electrode is represented by Kp and the initial efficiency of the negative electrode is represented by Kn. If the ratio Kp/Kn of the initial efficiency of the positive electrode to the initial efficiency of the negative electrode is smaller than 0.9 or larger than 1.1, there arises a problem that the efficiency/capacity of the battery using these electrodes decreases. In other words, the capacity of the battery will not be large even if only one of the positive and negative electrodes has a large initial efficiency. In the present invention, it is more preferable that Kp and Kn satisfy the relationship: 0.95xe2x89xa6Kp/Knxe2x89xa61.05.
The initial efficiency of each of the positive and negative electrodes can be adjusted in accordance with the characteristics of the active material itself. For example, in the positive electrode, the initial efficiency will decrease if the value of x in LixNiyMzO2 increases. Also, the initial efficiency can be increased by reducing the amount of defects in the crystal. In the negative electrode, the initial efficiency can be adjusted by attaching, on the negative electrode surface, a substance that reacts with lithium to form a compound. The initial efficiency will also change in accordance with the specific surface area, the shape, and the calcining condition of the active material. As for the internal structure, the initial efficiency can be changed by introducing an element other than carbon to the inside. Also, the initial efficiency can be adjusted by mixing a material having a different initial efficiency and changing its mixing ratio.
The initial efficiency of each of the positive and negative electrodes can also be adjusted in accordance with the kind and amount of the electrically conductive agent, the amount of the binder, the pressure in the calendering process, the degree of dispersion in the active material mixture-coating material, and the like. For example, the initial efficiency of the electrode can be increased by increasing the amount of the electrically conductive agent while avoiding decrease in the electrode capacity, by decreasing the amount of the binder while maintaining the strength and the adhesion of the active material layer, or by raising the pressure in the calendering process. The reverse of the above-mentioned adjustment can be carried out in order to decrease the initial efficiency of the electrode.
The non-aqueous electrolyte secondary battery of the present invention employs an electrolytic solution obtained by dissolving, in an organic solvent, a lithium salt to be used as a supporting electrolyte.
The organic solvent to be used is not specifically limited. As the organic solvent, propylene carbonate, ethylene carbonate, dimethoxy ethane, xcex3-butyrolactone, tetrahydrofuran, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, and the like are used either alone or as a mixture of two or more kinds thereof.
The supporting electrolyte to be used is not specifically limited. As the supporting electrolyte, LiClO4, LiAsF6, LiPF6, LiBF4, and the like are used either alone or as a mixture of two or more kinds thereof.
The non-aqueous electrolyte secondary battery can have a variety of configurations. For example, besides the coin-type battery, the battery can have a configuration such that a jelly roll prepared by using a positive electrode, a negative electrode, and a separator is housed in a round or square can.